In electrical discharge machnining, it has been commonly recognized that a hydrocarbon liquid such as kerosene or transformer oil provides a highly favorable machining medium which can be introduced or supplied into the machining gap under pressure to serve as the discharge medium, the coolant and the flushing medium. Unfortunately, however, such hydrocarbon liquids are inflammable and must be used with anti-fire, fire-alarm and fire-fighting equipment.
The danger of a fire is entirely eliminated by the use of a water liquid, which not only has widely been used in traveling-wire EDM but is sometimes exploited in sinking-type EDM as well. In addition to its noninflammability, it is readily available, is inexpensive and entails a minimum problem of disposal.
There is, however, a problem with a water machining liquid. Water is less satisfactory in stock removal and machining stability than a hydrocarbon such as kerosene. In the use of water, machining becomes unstable and the removal rate drops noticeably when the duration of discharge pulses is longer than 30 to 50 microseconds. As the pulse duration is lengthened, an arc discharge more likely ensues with a water liquid having a specific resistance in the range between 10.sup.3 and 5.times.10.sup.5 ohm-cm than with a hydrocarbon liquid. Furthermore, some electrolytic actions are inherent and cannot entirely be excluded when an electric current is passed through such a water machining liquid, thus limiting the extent of dielectricity and dielectric gap recovery which are essential in the EDM process and consequently limiting the desired machining stability. In addition, it should be noted that machining instability with water is attributed to its excessive cooling ability.